Recently, a thin and stylish mobile phone, which has a sound function such as a video telephone, moving image reproduction, or a handsfree telephone function having a commercial value, has been actively developed. Thereby, an electro-acoustic transducer, which is mounted on a mobile phone or the like, is also required to be small in the size and to reproduce sound with a very loud volume. Moreover, from the standpoint of privacy protection, development of an ultra-directional speaker, in which a sound field can be formed in only a specific position, is required. As the ultra-directional speaker, a parametric speaker, which demodulates modulated ultrasonic waves by a nonlinear state in air, has been developed.
The parametric speaker oscillates an ultrasonic wave oscillator by a single frequency. Thereby, it is preferable that the speaker be configured by a structure having a high mechanical quality factor. Therefore, energy is concentrated near a resonance frequency, and a sound can be reproduced with high efficiency.
Currently, various mobile phones are suggested as the above-described mobile phone (Patent Document 1).